Hello! Welcome back to our exploration of the Hydrosphere.
In our last lesson, we investigated how ocean temperature and salinity vary across the globe. We saw that solar energy heats the surface water, especially at the equator, while processes like evaporation, precipitation, and ice formation create different levels of saltiness. You learned that cold water is denser than warm water, and salty water is denser than fresh water.
Today, we'll see the direct consequence of these variations. This lesson fulfills the learning outcome: Analyze the major surface and deep-ocean current systems and the forces that drive them. We will explore how the ocean is in constant motion, driven by two distinct engines: one at the surface powered by wind, and another in the deep ocean powered by density. These currents act as the Earth's circulatory system, transporting vast amounts of heat and shaping global climate patterns.
1. An Overview of Ocean Motion
Before diving into the details, let's get a big-picture view of how global winds and ocean currents are connected.
Global winds and currents | Middle school Earth and space science | Khan Academy
This Khan Academy video provides an excellent conceptual overview, linking uneven solar heating to winds, which then drive surface currents. It also introduces how temperature and salinity drive deep currents, setting the stage for our entire lesson.
Watch the full video (up to 05:26). Pay close attention to how it connects several concepts: 1) The sun's energy creates wind. 2) Wind and the Coriolis effect create surface currents called gyres. 3) Temperature and salinity differences create density-driven deep currents (the 'overturning circulation').
As the video explained, we can broadly divide ocean circulation into two main types based on their driving forces and location:
- Surface Currents: Occur in the upper few hundred meters of the ocean and are primarily driven by wind.
- Deep Currents: Occur throughout the rest of the water column and are driven by differences in water density.
Let's analyze each of these systems in more detail.
2. Surface Currents: A Wind-Driven System
Surface currents are the fast-moving "rivers" of the upper ocean. They are powered by a combination of forces, with wind being the primary source of energy.
The Smithsonian Ocean Portal article 'Currents, Waves, and Tides' provides a fantastic, detailed explanation of the forces that create surface currents. We will read the sections that break down these drivers.
Read the sections 'Surface Currents', 'Sun and Wind', and 'Gravity and Earth's Rotation'. Focus on understanding the chain of events: The Sun's uneven heating creates global wind patterns. Wind drags the ocean surface through friction. This motion is deflected by the Coriolis effect. Water piles up into 'hills', and gravity's pull on this water, also deflected by Coriolis, creates large rotating currents.
Let's summarize the key forces you just read about:
- Wind: As you learned in the atmosphere modules, persistent global wind systems like the Trade Winds and the Westerlies blow over the ocean. Through friction, they drag the surface water, transferring energy and setting the water in motion.
- Earth's Rotation (Coriolis Effect): This is the same force that deflects winds. It causes moving water to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This prevents currents from simply flowing in the same direction as the wind.
- Gravity: Wind doesn't just drag water; it can also push it and pile it up, creating subtle "hills" and "valleys" on the ocean surface. Gravity then tries to pull the water from the high areas (hills) back down to the low areas (valleys).
- Landmasses: The continents act as barriers, blocking the flow of water and forcing the currents to turn and create closed loops.
The Result: Ocean Gyres
The interaction of these forces—wind pushing, Coriolis deflecting, gravity pulling, and continents blocking—organizes surface currents into massive, circular-moving loops of water called gyres.
There are five major ocean gyres:
- North Pacific Gyre
- South Pacific Gyre
- North Atlantic Gyre
- South Atlantic Gyre
- Indian Ocean Gyre
Observe the map closely. You'll see a consistent pattern emerge, which is a direct result of the Coriolis effect:
- In the Northern Hemisphere, gyres rotate clockwise.
- In the Southern Hemisphere, gyres rotate counter-clockwise.
This rotation has a significant impact on coastal climates. On the western side of an ocean basin (e.g., the U.S. East Coast), currents bring warm water from the equator poleward. On the eastern side of a basin (e.g., the U.S. West Coast), currents bring cold water from the poles toward the equator.
Test your understanding!
Consider the North Atlantic Gyre. Japan is located on the western edge of the North Pacific basin, and California is on the eastern edge. Using the principle of clockwise gyre rotation in the Northern Hemisphere, explain what kind of current (warm or cold) you would expect to find along the coast of Japan versus the coast of California.
Show answer
- Japan: The North Pacific Gyre rotates clockwise. On the western side of the Pacific (near Japan), the current flows northward from the tropics. This is the warm Kuroshio Current.
- California: On the eastern side of the Pacific (near California), the current flows southward from the polar regions to complete the loop. This is the cold California Current.
This explains why, despite being at similar latitudes, coastal waters and climates can be so different.
A Deeper Look: Ekman Transport
An interesting consequence of the wind-Coriolis interaction is a phenomenon called Ekman Transport. When wind blows over the surface, the topmost layer of water moves at about a 45° angle to the wind (due to the Coriolis effect). This layer then drags the one below it, which moves a bit slower and at an angle to the first layer. This creates a spiral effect down the water column. The net result is that the overall movement of the water column is about 90° to the direction of the wind. This process is crucial for causing upwelling and downwelling, which we will touch on in future lessons.
3. Deep Currents: The Great Ocean Conveyor Belt
Beneath the wind-driven surface currents lies a slower, deeper, and even larger circulation system. This system is not driven by wind, but by small differences in water density. Because temperature (thermo) and salinity (haline) control density, this is called thermohaline circulation.
This article from Penn State's Earth 103 course clearly explains the thermohaline circulation and introduces the 'Global Conveyor Belt'.
Read the sections starting from 'The surface currents described above...' to the end. Focus on understanding: What thermohaline circulation is. The two main places where dense, deep water forms: the North Atlantic (NADW) and near Antarctica (ABW). The processes that make the water dense enough to sink (cooling and increasing salinity).
The Engine of the Deep Ocean
As you just read, the entire deep ocean is put into motion by water sinking in just a few, very specific locations.
- North Atlantic Deep Water (NADW) Formation: Warm, salty water from the tropics flows northward via the Gulf Stream. When it reaches the high latitudes of the North Atlantic (near Greenland and Iceland), it encounters frigid polar air. It cools dramatically, becoming very dense. At the same time, some water may freeze into sea ice, leaving the remaining water even saltier and denser. This cold, salty, dense water sinks to the ocean floor.
- Antarctic Bottom Water (ABW) Formation: A similar process happens around Antarctica. As sea ice forms, the salt is excluded from the ice, making the surrounding water extremely salty and cold. This water becomes the densest in the entire ocean and sinks to the very bottom.
This sinking of dense water in the polar regions pushes the deep water along the ocean floor, driving a massive, slow, global circulation system known as the Global Conveyor Belt.
The conveyor belt works like this:
- Deep water formed in the North Atlantic flows south.
- It joins with the Antarctic Bottom Water and circulates around Antarctica, branching off into the Indian and Pacific Oceans.
- As it travels, it slowly mixes and warms, becoming less dense.
- Eventually, it rises back to the surface (a process called upwelling), mainly in the Pacific and Indian Oceans.
- Once at the surface, it joins the wind-driven surface currents, eventually making its way back to the North Atlantic to complete the loop.
This entire global circuit takes roughly 1,000 years to complete. It acts as a massive heat distributor, and by carrying dissolved CO2 into the deep ocean, it plays a critical role in regulating Earth's climate.
Conclusion
In this lesson, you've analyzed the two major systems of ocean circulation and the forces that drive them. You have seen that the ocean is not a static body of water but a dynamic system in constant motion, fundamentally shaping our planet's climate.
Key Takeaways:
- Ocean circulation is divided into surface currents and deep currents.
- Surface currents are found in the upper ocean and are driven primarily by wind, with their direction shaped by the Coriolis effect and continental barriers. They are organized into large rotating gyres.
- Deep currents are driven by differences in density, a process known as thermohaline circulation.
- Dense water forms when it becomes very cold and/or very salty. This happens mainly in the high-latitude polar regions (North Atlantic and Antarctica), where the water sinks.
- This sinking drives the Global Conveyor Belt, a slow-moving, worldwide current system that mixes the entire ocean over a timescale of about 1,000 years.
Preview of the Next Lesson:
We have just examined the long-term, average state of ocean circulation. However, this system can vary significantly from year to year, with dramatic consequences for global weather. In our next lesson, we will focus on the most important of these variations: "Explain the formation and climatic impacts of the El Niño-Southern Oscillation (ENSO) phenomenon." We'll see how a temporary change in winds and currents in the Pacific Ocean can lead to floods, droughts, and unusual weather all around the world.